I've got a pair of Marti STL-10 / R10 systems here around 949 MHz. Making 10 watts into a good dummy load.

I connected the RF output to a 50w 30dB coaxial attenuator followed by a 2w 20dB coaxial attenuator, to the input of my spectrum analyzer. The SA claims the 948 MHz carrier is at -9.75dBm, which is close enough for me. 10w is +40dBm and I've got 50dB of attenuation so as to not overload the SA. -10.0dBm would be the target.

These units generate and amplify the signal at 1/2 the desired output frequency. They then run it through a varactor doubler to come up with the 950 MHz signals.

There's a sub-harmonic at 474 MHz that's at -66dBm, which is about -56dBc. The Marti specs call for -60dBc. I also checked up around 1422 MHz (3 times 474 MHz) and found a similar -66dBm spike. My SA is only good to 1.5 GHz so I can't check any higher.

Both units have the same levels at the output frequency and +/- 1/2 the output frequency. Either they're both tuned wrong or that's just how they are.

Is this normal? Suggestions to minimize the out-of-band signals besides some sort of bandpass filter? Does anyone have anything to compare this to?

If an XMTR does not make published spec, it's either your test equipment/setup, a problem with the XMTR, or specmanship. We use STL-10's for an AM link but I've never put them on an analyzer.

To allow folks to sleep easier, consider that the antenna likely shows loss at half-freq, so the sub at 474 MHz is likely radiating beneath allowable spec limit. Could be the same for the 2nd harmonic, if the antenna is selective enough.

If I had this unit on the bench, I'd look closely at tuning around the doubler and PA, while operating into a known 50 Ohm dummy. Lots of things can cause a dirty signal; poor connections, bad power supply bypass, RF feedback, loose screws on a shielded enclosure, etc.

I feel reasonably confident that with the 50dB of attenuation ahead of the SA, the harmonics I was seeing are truly present. When I increased the input signal by removing some attenuation, the fundamental and all harmonics increased by exactly the same amount. Usually SA overload or internally-generated spurs will not change by the same amount as the fundamental when the signal level is changed, especially by 10-20dB.

If all else fails & you don't have the time to bench it, I'd recommend Bohn Broadcast. I sent 3 pairs of those STLs out to them & they came back like new... better power supplies, replaced rotary switches, LEDs for the meter lamps & a bunch of updated parts.

I know there are others who do this kind of work. In this case though, I couldn't have found the time to do the work these guys did for the price.

I didn't have enough loaner gear to take all 3 out at the same time, but a couple of companies out here have been replacing their 950 gear with internet transports and had extras on their shelves. Good to have friends.

The unit was on my bench over 3 years ago. It's either back in service working fine or sitting on a shelf until something else breaks and it gets put into service. I'm not losing sleep over a few dB which will probably be attenuated by the antenna system.